Nov. 17, 2015 10:43

Controlling Construction Site Runoff

Construction Runoff
"We’re excited to announce that  we’re breaking ground tomorrow on our brand-new (fill in the blank) development.” How many times have you heard that phrase? It points up the fact that construction of anything almost always involves ‘breaking ground,’ i.e., disturbing the earth in some fashion.

And when the earth is disturbed, there’s always the potential for losing some of it to wind, water or wheels. This runoff then has the potential of finding its way to local streams, lakes and even the ocean.

According to the U.S. Environmental Protection Agency (EPA), some of the pollutants commonly discharged from construction sites include: sediment, solid and sanitary wastes, phosphorous and nitrogen from fertilizers, pesticides, oil and grease, concrete truck washout, as well as chemicals and debris. Sediment alone is one of the biggest pollutants affecting watersheds, second only to pathogens.

In a short stretch of time, construction sites can contribute more sediment to streams than would have been deposited naturally over several decades. It can quickly fill them, requiring dredging, and destroying aquatic habitats.

Time was, we didn’t worry so much about construction-site runoff. However, since the early 1970s, laws such as the Clean Water Act have been put in place to make sure we do. Stormwater Pollution Prevention Plans (SWPPPs) have to be filed to make sure construction companies are compliant with the National Pollutant Discharge Elimination System (NPDES).

How hard is it to control this runoff? According to Dave Burns, co-owner of SWPPP Solutions in Redding, California, it can be “a cakewalk—or a nightmare. It varies widely. Some sites that are flat don’t have any issues. Others that have very steep slopes around them, or are next to creeks or wetlands, can be quite complicated.”

To do the job, an erosion-control contractor, construction company project manager, or whoever is assigned the task, can turn to a number of tried-and-true best management practices (BMPs).

Silt fencing

Composed of porous geotextile fabric and held up by wood or metal posts, this is the ubiquitous go-to solution found on nearly every construction site in the world. Whatever other runoff control measures may be in place, silt fencing is there to back them up. “The silt fence has always been the first line of defense,” said B.J. Frideres, project manager for the Weitz Company, a construction firm based in Des Moines, Iowa. “Especially when you’ve got a completely open site that’s just been graded.”

“When properly installed, silt fencing is one of the most, if not the most, effective temporary sediment control device out there,” said inventor and business consultant Thomas Carpenter, CPESC, owner of Erosion Tech, LLC, in sJuliette, Georgia.

A single 100-foot run of silt fencing can hold 50 tons of sediment in place. It has the ability to stand up to concentrated flows and strong winds, is inexpensive and can be installed fairly quickly.

Let’s go back to that phrase, “when properly installed.” When silt fence fails, improper installation is usually to blame. It’s supposed to be trenched in and backfilled with soil, and may have been, but perhaps not well enough or deeply enough.

Sometimes, trenches aren’t completely cleaned out prior to installation, and debris can interfere with installation and backfilling. If the fence isn’t inserted into the ground at a uniform depth along its entire length, these shallow areas can wash out. The backfill may not have been adequate, or not compacted enough.

“For this device to work, water has to pond up behind it,” said Carpenter. “The ponding is what allows the sediment to fall out. Even if you have some run-over, the heavier sediment will have settled out behind the silt fence. At the end of the rainstorm, that filled-up silt fence has kept a lot of sediment from getting into the watershed.”

Frideres can attest to this. “Silt fences do a heck of a job, to the point where you sometimes have to tear them out and put in new ones, because they’ve caught so much runoff, they’re completely full. These things will stay up until the very end of a project, when we’re ready to do some seeding. I can’t think of a job where I haven’t used silt fencing since all the environmental rules went into place.”

Silt fencings’ very effectiveness can be its worst enemy. “They work well, as long as they’re maintained,” said Adele Beaves, P.E., CPESC, senior project manager at Pittsburgh, Pennsylvania-based Civil & Environmental Consultants, Inc. “The problem with silt fencing—and I see this all the time—is that you’ll see one buried in sediment up to two inches from the top.”

“Sediment has to be removed when it reaches half the height of a silt fence, but you often see people not doing it. I’ve also seen where they’ve flattened them, run them over with machines,” she said.

Don’t let neglect spoil your silt fence. Clean it out or replace it when needed, and it will do the job effectively.

Straw Wattles

There are some very sophisticated BMPs for controlling construction site runoff. However, some are as simple and basic as straw. Straw wattles, also sometimes called straw noodles, worms, or bio-logs, are constructed cylinders of compressed, weed-free wheat or rice straw, usually eight to 12 inches in diameter, and 20 to 25 feet long. The outer casing is nylon, jute, or other photo- or biodegradable materials.

When installed, usually in shallow trenches along the contours of slopes or around the perimeters of construction areas, they slow down water and sediment very effectively. How effectively? They’re often used as BMPs in fire areas and on slopes that have less than 30 percent of the original ground cover remaining.

Straw wattles add roughness, increase infiltration, and help retain soil. They’re lightweight, usually around 35 pounds, are easy to install, and are one of the most cost-effective sediment control solutions available. Lasting up to two years, they can be removed after a project is done, or, when covered in biodegradable material, left in place.

However, they shouldn’t be used in areas of concentrated flow, as they’re simply not sturdy enough to stand up to such forces. They shouldn’t be laid across drainage swales or channels with more than two acres of contributing drainage area.

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